Skin is made up of several layers of cells which coat and protect the keratin and collagen fibrous proteins that form the skeleton of its structure. The outermost of these layers, referred to as the stratum corneum, is known to be composed of 250 .ANG. protein bundles surrounded by 80 .ANG. thick layers. Anionic surfactants and organic solvents typically penetrate the stratum corneum membrane and, by delipidization (i.e. removal of the lipids from the stratum corneum), destroy its integrity. This destruction of the skin surface topography leads to a rough feel and may eventually permit the surfactant or solvent to interact with the keratin, creating irritation.
It is now recognised that maintaining the proper water gradient across the statum corneum is important to its functionality. Most of this water, which is sometimes considered to be the stratum corneum's plasticizer, comes from inside the body. If the humidity is too low, such as in a cold climate, insufficient water remains in the outer layers of the stratum corneum to properly plasticize the tissue; and the skin begins to scale and becomes itchy. Skin permeability is also decreased somewhat when there is inadequate water across the stratum corneum. On the other hand, too much water on the outside of the skin causes the stratum corneum to ultimately sorb three to five times its own weight of bound water. This swells and puckers the skin and results in approximately a two to three fold increase in the permeability of the skin to water and other polar molecules.
Thus, a need exists for compositions which will assist the stratum corneum in maintaining its barrier and water retention functions at optimum performance in spite of deleterious interactions which the skin may encounter in washing, work, and recreation.
Conventional cosmetic cream and lotion compositions as described, for example, in Sagarin, Cosmetics Science and Technology, 2nd Edition, Vol. 1, Wiley Interscience (1972) and Encyclopaedia of Chemical Technology, Third Edition, Volume 7 are known to provide varying degrees of emolliency, barrier and water-retention (moisturising) benefits. However, they can also suffer serious negatives in terms of skin feel (i.e. they often feel very greasy on the skin) as well as having poor rub-in, absorption and residue characteristics.
It would also be desirable to provide a cosmetic composition which delivered pigments to the skin of the user at the same time as providing excellent moisturisation. A pigmented cosmetic composition can serve to even skin tone and texture and to hide pores, imperfections, fine lines and the like.
At the same time, it would be desirable to provide a moisturising composition having topical anti-acne activity. There are many compounds which are known to exhibit anti-acne properties when applied topically to the skin. A commonly used keratolytic agent having anti-acne activity is salicylic acid. As salicylic acid is virtually insoluble in water however, it is difficult to incorporate into the aqueous phase of an emulsion composition. Delivery of salicylic acid from the pigment-containing oil phase of an emulsion composition can, on the other hand, lead to discolouration of the composition due to interaction between the salicylic acid and pigments, especially of the iron oxide type. Furthermore, in compositions requiring high water levels, such as skin moisturising compositions, salicylic acid tends to precipitate out of solution. It would therefore be desirable to deliver salicylic acid in soluble form from the aqueous phase but without the salicylic acid precipitating out of solution.
It is accordingly one object of the present invention to provide a cosmetic composition which exhibits anti-acne activity together with moisturisation benefits.
It is a further object of the invention to provide a cosmetic composition in the form of a multiple phase emulsion having improved product and colour stability.